Grid is
a type of a digital terrain model which consist of elevations taken at
regularly spaced intervals in two horizontal coordinate directions. These two horizontal directions should
coincide with the northing and easting of the specified project coordinate
system. Grids are advantageously used
for measuring original and final terrain of borrow areas for quantity purposes.

Cross
Sections are measured perpendicular to and at regular intervals and on low and
high points along a centerline or baseline.
Each cross section or baseline, measured for each significant break in
the terrain. This type is used for
defining the original terrain along the proposed line for the highway.

Re-measurement
defines the terrain after earthwork is completed. Original measurement may have been in grid or cross-section
pattern; re-measurement extends only as far as construction operations changed
the terrain, and it includes measurement of the same grid points or along the
same cross section lines, plus significant breaks in the surface as altered by
construction.

Critical
Points define the topography of an area using three coordinate. From such data
other descriptions of the topography can be derived, such as contours or cross
sections along any chosen alignment.

The
digital terrain model shall be grid, cross section, re-measurement or critical
point type, whichever will be specified and applicable to the particular
project. For grid digital terrain
models, the maximum spacing shall be fifteen (15) meters, and likewise the same
maximum space for cross section, re- measurement or critical points. For cross
sections the spacing shall be measured along the centerline or base line.

NJDOT
shall show on a map the area to be included in the digital terrain model by
outlining it or by drawing a centerline
or base line and specifying the width of coverage. A written description of the
area in question may further define the area.

All mapping must be compiled strictly in accordance with
the requirements set forth in these specifications.

All digital data shall be recorded directly as a function
of stereoplotter operation. The operator should not read the stereoplotter
settings and transfer them to the media.
Post compilation digitizing or graphic compilation will not be
permitted.

The Contractor must submit the name and type of digital
photogrammetric instrument to be utilized along with proof of the
manufacturer's calibration which shall have been performed within the past two years.

All digitally compiled mapping must be 100% clean and
digitally edited by interactive editing equipment; "electronic
manuscripts" will not be an acceptable delivery.

Submittable hardware and software formats shall be
verified with NJDOT and the client by the photogrammetrist on a project by
project basis.

All files shall be two-dimensional files. Coordinate values for all features shall be
based on the grid system indicated by the control data.

Files shall be compiled with coordinate values to the nearest
millimeter (MM). If supplying
INTERGRAPH design files to the NJDOT, the working units shall be as follows:

Master Units = meter (M)

Sub Units = millimeter (MM)

Resolutions = 10 positional units per millimeter (MM)

To keep project files at manageable sizes, maximum file
sizes shall be verified with NJDOT and the client by the photogrammetrist on a
project by project basis and contain planimetric features for one or more
entire stereo models. Individual stereo
models shall not be separated into more than one file. Contours shall be in separate files
corresponding in area to each planimetric file.

In obscured areas where the photogrammetrically measured
digital terrain model cannot be completed by this method, ground surveys shall
be employed to complete the work.

The
Root-Mean-Square-Error (RMSE) which is defined as the square root of the
quotient of the sum of the squares of the errors divided by the number of
measurements, or

RMSE =
[(S e2)/n]1/2

in which
“e” is the error at each point (the difference between the value used as a standard and the value
being tested) and “n” is the total number of points tested. The elevations, in meters, of all points
tested shall not exceed the limit specified by NJDOT for the work. (Refer to Section 6-06 to discussions and
procedures for the selection of RMSE limits). No individual error shall exceed
three(3) times the specified limit for root-mean-square error. The average error (the algebraic sum of the
individual test point errors, taking into consideration their signs, divided by
the number of test points) shall not exceed three-tenths the specified limit
for root-mean-square error.

The
root-mean-square- error and the average error for any individual cross section
or any individual group of grid points shall not exceed twice the limits
established for the entire digital
terrain model.

The
first consideration for a photogrammetrically produced digital terrain model is
the relationship of the average depth of excavation and the needed
accuracy. The average depth of
excavation is the square meters of surface in the excavation areas divided by
the number of cubic meters of materials to be excavated in the same areas. The root-mean-square-error for pay
quantities should not exceed the average depth of excavation multiplied by the
factor 0.03, but even in the heaviest excavation, the root-mean-square-error
should not exceed 0.30 meters. If preliminary quantities only are being
computed which will not be used for pay, the factor may be relaxed to 0.10 and the maximum to 0.45 meters. With the
average depth of excavation estimated,
this gives the root-mean-square-error to specify for ground surveyed digital terrain models and serves as a guide
as to limits of usefulness of the photogrammetric method.

The
second consideration for photogrammetrically produced digital terrain models is
the capability of the stereoplotters.
On open ground (no interfering cover)
with uniform slopes, a double projection stereoplotter should be capable
of producing elevations with a root-mean-square-error as small as 1/6 000 of
the flight height; and an optical train stereoplotter, as small as 1/10 000 of
the flight height. With irregular
slopes, these reduce to about 1/5 000 and 1/8 000, and, with interfering ground
cover, to about 1/3 000 and 1/5 000.
This tells us that, considering the minimum flight height of 270 meters
satisfactory pay quantities can be produced by a double projection instrument
if the average depth of excavation is as small as (270/(6 000 x 0.03) = 1.5
meters, and by an optical train stereoplotter if the average depth of cut is as
small as (270/10 000 x0.03) = 0.90 meter.
For preliminary quantities for design, the minimum depth of excavation
using a double projection
stereo-plotter is as small as (270/6 000 x 0.10) = 0.45 meter, and using an optical trained stereoplotter, as small
as (270/(10 000 x 0.10) = 0.27 meter.
Photogrammetric quantities are usually more accurate and less costly for
moderate and heavier work, but for very light work, ground surveys are
necessary.

Since
average depth of cut is only an estimation when planning a photogrammetric mission, and flight height
may be the quantity to define, a third consideration is the accuracy required for the use to be made of the quantities and types of
terrain. Pay quantities where the
slopes are uniform, i.e. flat to rolling terrain, require a root-mean-square-error
of 0.06 meter or less, and where slopes are irregular, i.e. rugged terrain,
require a root-mean-square error of 0.20 meter or less. For preliminary quantities, flat to rolling
terrain with no interfering ground cover requires a root-mean-square-error of
0.20 meter or less, and rugged terrain with interfering cover requires 0.50
meter or less.

A
digital terrain model for computing earthwork quantities for pay shall not be
accepted until at least four (4)
percent of the grid points or of the cross sections have been compared
with ground surveyed data and have met specifications requirements. Grid points
shall be in groups of 20 to 40 contiguous points. Cross sections shall be complete sections. No more than two (2)
adjacent sections may be included at any location. These single sections, pairs of sections, or groups of points shall
be distributed over the project area so as to be representative of the whole
digital terrain model. A digital terrain model for computing preliminary
earthwork quantities for design only may be accepted when the agency has made
enough ground surveyed or stereoplotter tests to assure that the work meets all
specification requirements.